The invention pertains to an optical device making it possible to obtain an image of an observed scene and, at the same time, the optical spectrum at any point of said image.
Such a device allows in particular the discrimination of elements present in a scene, whether gasses, liquids, solid materials or particles.
A first approach, known from the prior art, for obtaining optical image spectra (or spectral images) consists in disposing narrow passband filters in front of a camera, in order to acquire substantially monochromatic images, consisting of light rays having the same wavelength; by performing a plurality of image acquisitions of the same scene using different filters it is possible to reconstitute the optical spectrum of each point of said scene. Such an approach is not satisfactory for several reasons. Firstly, the luminous flux gathered by the camera is generally too weak to make it possible to achieve an acceptable signal-to-noise ratio; an increase in the flux can be obtained only by widening the passband of each filter, that is to say by degrading the spectral resolution. This problem is particularly serious in the case of commercial applications, in which it is desired to use uncooled radiation sensors so as to reduce the cost and complexity of the device. Another drawback is due to the fact that the luminous flux gathered varies greatly from one wavelength to another: consequently the sensitivity and the operating point of the radiation sensors vary from one acquisition to another.
A more promising approach, also known from the prior art, consists in using, in place of the narrow passband filters, band rejection filters, the rejected band also being narrow. A substantially monochromatic image at the wavelength λ1 can be obtained by subtracting from a reference image acquired without any filter, an image acquired through a filter eliminating the spectral component at this same wavelength λ1. This technique makes it possible to obtain a better signal-to-noise ratio and more contained variations in luminous flux, but it nevertheless exhibits numerous drawbacks related to the use of filters: the wavelengths stopped by the filters cannot be varied in a continuous manner, this leading to a sub-sampling of the spectral space; each filter exhibits, outside the spectral rejection band, a different transmission curve; the use of filters of different thicknesses leads to geometric shifts between the various images; and the movement of the supports of the filters past the sensors tends to modify the operating point of the latter.
For these reasons, the present inventor has developed an optical device using a continuously tunable spectral rejection device not exhibiting the aforesaid drawbacks. This device is described in document FR 2 788 137 and in the article:
Yves Guern, Laurence Grenier and François Carpentier, “Uncooled IRFPA for low-cost multispectral/hyperspectral LWIR imaging device”, Spie, Vol. 5093, April 2003, page 126.
This device comprises in particular a first spectral dispersion means (grating, prism), a mask interposed in the path of the dispersed rays so as to intercept a narrow band of wavelengths and a second spectral dispersion means (grating, prism), acting as means for recombining the light rays dispersed by the first means. A continuous scan of the spectrum can be obtained by displacing the mask.
This device constitutes a considerable improvement with respect to the prior art, but it nevertheless exhibits certain drawbacks.
A first drawback is related to the presence of two spectral dispersion means: this gives rise to flux losses and a reduction in the optical passband. Furthermore, diffraction gratings and prisms are expensive devices.
Another drawback consists in the fact that the optics of the device are relatively difficult to design for reasons which will become clearer subsequently.
Another drawback is related to the fact that any modification of the scene or any movement of the apparatus in the course of a series of image acquisitions is liable to generate artifacts in the information acquired, which is compensated at the price of more complex information processing.